1. Field of the Invention
The present invention relates to a signal processing circuit for a vector scope adapted to measure the characteristics of a color video signal, and more particularly to a signal processing circuit adapted to automatically set at least one of the amplitudes represented by vectors of a burst signal and a chroma signal at a specified amplitude position on a display screen.
2. Prior Arts
A vector scope in a measuring instrument for demodulating an input color video signal to display in terms of a vector the relation between R-Y and B-Y chrominance components containing color information and being capable of measuring and displaying the characteristics of the color video signal. The angle and amplitude of the displayed vector are related to the hue and the chroma of the color represented by the color information. The display screen of the vector scope is represented as shown in FIG. 1 for example. In FIG. 1, the vertical axis indicates the R-Y component while the horizontal axis indicates the B-Y component. The graduation "a" provided along the circumference of the display screen serves to indicate a phase of the chroma signal contained in the color video signal and is capable of indicating a jitter component contained in a burst signal. The graduation "b" provided at the leftward and of the horizontal axis serves to indicate a differential gain (DG) and a differential phase (DP) of the chroma signal.
In use when a jitter component of a burst signal is desired to be measured, the tip end of a vector representation V.sub.b on the display screen which represents the burst signal must be aligned with the graduation "a", or a circumferential position of the display screen. According to a prior art, a lug on a front manipulation panel is manually operated to control a variable gain circuit incorporated in a signal processing circuit, watching the display screen as to align the tip end of the vector representation of a burst signal with a circumferential position of the display screen.
In the case that a DG or DP of the chroma signal is to be measured, it is necessary to adjust the tip end of a vector representation V.sub.c of the chroma signal to the reference point 0 (the crossing point of the horizontal axis and the circumference). Also in the case, by manually handling lugs of the manipulation panel to control a gain and phase (angle) of the chroma signal, watching the display screen, the tip end of the vector representation of a chroma signal may be adjusted to the reference point 0 of the graduation "b".
Although such a signal processing circuit has already been proposed as a vector representation of a burst signal which may be automatically aligned with the horizontal axis, even such a prior art processing circuit requires an amplitude of the vector representation to the manually adjusted and an amplitude and angle of a vector representation of a chroma signal cannot be automatically adjusted to given values.
As explained above, according to the prior art, the amplitude and/or the angle of the vector representation have to be adjusted to and aligned with the given values by manually handling the lugs of the manipulation panel, watching the display screen. As the consequence, the operation is cumbersome and it is difficult to align the vector representation accurately with the specified positions (in respect of the amplitude and angle).